The present invention relates to a reactive plugging fluid designed to gel rapidly when subjected to high shear stress. The invention also relates to a method for plugging a subterranean formation zone, especially for curing massive mud losses when drilling a well.
Lost circulation while drilling is a major problem. The well cost dramatically increases due to the lost time from delayed well production and also due to associated drilled problems such as pipe sticking and safety issues. The most common technique to combat lost circulation is to add into the drilling fluid a lost circulation material (LCM). Granular flakes and fibrous particles, essentially based on cellulosic materials, are used for sealing off fractures, vugs and porous zones. Minerals such as mica are also commonly used. If even high concentrations of lost circulation materials fail to restore the drilling fluid circulation, a cement plug is placed. The cement plug consolidates the voids but also fills the open wellbore and needs to be drilled before continuing the well drilling. Quite often, the procedure must be repeated several times before achieving a correct seal.
Other techniques involve the use of reactive fluids. Two reactive fluids are either mixed near the formation where lost occurs with a first fluid pumped through the drill-string and a second fluid displaced down the annulus. At the interface of the two fluids, the turbulent flow allows the rapid formation of a rubbery solid mass commonly known as a gunk. Another practice uses crosslinked polymer gels whose reaction is initiated on surface. In both case, the technology is highly risky since slight changes in the composition, temperature or fluid contamination may lead to premature gellation in and around the bottom hole assembly, leading to major operation failure.
It is also known to use as plugging fluids so-called rheotropic liquids that thicken when subjected to high stress. U.S. Pat. No. 4,663,366 discloses such a polycarboxylic acid containing water-in-oil emulsion where the oil phase contains hydratable water-swelling hydrophilic clay such as bentonite and the aqueous phase contains a dissolved polyacrylamide and a polycarboxylic acid. The setting of this plugging fluid takes place as a result of a swelling of the bentonite when bentonite contacts water. Each dispersed droplet of the aqueous phase is coated with a polymeric material so that the contact only occurs when the emulsion is subjected to high shear forces that break this coating.
Another rheotropic plugging fluid is known from U.S. Pat. No. 5,919,739 (Sunde et al.). Like the emulsion of the U.S. Pat. No. 4,663,366, the fluid is based upon a xe2x80x98loosexe2x80x99 invert emulsion. The continuous phase provides an encapsulation medium for a crosslinker and the internal phase consists of a high concentration of a polymer while the interfacial tension between the two phases is maintained by a concentration of a lipophilic surfactant.
A preferred plugging fluid of the Sunde et al. patent application consists of about 25% by volume of a continuous phase containing an hydrophobic liquid selected from mineral oils, vegetable oils, esters and ethers, an emulsifier on a triglyceride basis, bentonite and calcium hydroxide and of about 75% by volume of a dispersed aqueous phase containing water, xanthane and optionally, a weighting material such as barite. When this type of fluid experiences a significant pressure drop, an inversion of the emulsion occurs and the crosslinker is released into the aqueous phase resulting in the formation of a gel.
This latter type of plugging fluid can be stored for several weeks without reacting and pumped with a centrifugal pump for several hours. Gellation is fast and triggered only by subjecting the plugging fluid to high shear forces, for instance when forced through the drill bit. However, the use of this type of plugging fluid is limited by lack of robustness and shrinkage over time. Moreover, above a temperature threshold of about 90xc2x0 C., the gel becomes less rigid and turns into a viscous fluid due to the breaking of the crosslinked bonds.
It would therefore be desirable to provide a new plugging fluid suitable to effectively seal off the problem zone and stable across a wide temperature range. There is also a need in well control for better procedures, including placement strategies to help in making jobs successful.
Thus, the invention provides a plugging fluid for plugging a subterranean formation zone surrounding a drill hole consisting of an emulsion comprising a dispersed aqueous phase containing an aqueous base, and a continuous hydrophobic phase containing a hydroxyethylcellulose derivative graft polymer, a surfactant and a crosslinking activator of the hydroxyethylcellulose polymer.
The emulsion is believed to be invert (water-in-oil) though it might actually be direct (oil-in-water) with further water droplets within the large oil droplet, i.e. an invert emulsion in a direct emulsion.
The grafter polymers useful to carry out the invention are cellulose ether derivative with vinyl phosphonic acid grafts.
The cellulose ether derivative is preferably a hydroxyalkyl cellulose where the alkyl is selected from the group of ethyl and propyl. The preferred hydroxyalkyl cellulose is 2-hydroxyethylcellulose. A process for preparing cellulose ethers having at least one phosphorous containing substitute is known from U.S. Pat. No. 4,413,121, which is hereby referred to and incorporated by reference.
The principle of the setting of the plugging fluid of the present invention is essentially the same as for the plugging fluid of U.S. Pat. No. 4,663,366 discussed above. It is the crosslinking of the grafted hydroxyalkyl cellulose that causes the gel formation. Crosslinkable cellulose derivatives are known as state-of-the-art polymeric viscosifiers used in the oil industry in particular for controlling fluid loss in subterranean formations. Reference is made for instance to U.S. Pat. Nos. 5,439,057 and 5,680,900. A crosslink bond is created between a metal ion and the pendant groups along the polymer chain of the polysaccharides. Upon exposing the plugging fluid to a pressure drop greater than 2 MPa over a small dimension, it is believed that the emulsion inverts or flips from its invert state into a more stable direct state. The rupture of the emulsion droplets releases the crosslinker activator into the water phase thus providing metallic divalent ion to crosslink with the cellulose ether derivatives and forming the gel structure.